MOUNT WILSON OBSERVATORY. 219 



The measurement of over 600 standards of wave-length in the spec- 

 trum of iron has been completed by I\Ir. Babcock with the Fabry-Perot 

 interferometer for the region X3370-X6750, and the work is being carried 

 toward longer and shorter wave-lengths. Mr. St. John has covered 

 the same region with the higher dispersion of a long-focus grating spec- 

 trograph, which permits the wave-lengths of a much greater number of 

 lines to be measured. The results will be published together, as the 

 errors of these two methods are complementary. It is expected that 

 the excellent agreement of the results will help to eUminate the diffi- 

 culties that have delayed the adoption of international standards of 

 wave-length. Several American physicists are arranging to cooperate 

 with the international committee, of which Mr. St. John is chairman, 

 and a large number of wave-length determinations will be made by 

 both methods (pp. 228, 259, 260). 



The measurement of solar wave-lengths with the highest precision 

 is an operation demanding the most rigorous precautions, especially 

 in view of the errors ordinarily involved in the comparison of two 

 light-sources. For this reason the use of four different methods of ob- 

 servation (p. 261), and especially the pains that are being devoted to 

 fixing the exact positions of telluric and iodine standards for comparison 

 with solar lines, are of special importance. Not only the Einstein 

 effect, but many other questions, are in the balance, and extensive 

 laboratory investigations are necessary in dealing with them. Thus 

 the Fabry-Perot interferometer is being employed to measure the wave- 

 lengths of lanthanum, barium, calcium, cerium, and strontium lines, 

 and displacements caused by pressure and by pole-effect in the arc are 

 being determined with the highest possible precision (p. 261). 



Other laboratory investigations have included the determination of 

 the wave-lengths of oxygen and nitrogen lines given by metallic sparks 

 in the red and infra-red (p. 260), photographic observations of the 

 spectra of condenser discharges in high vacua (p. 263), and various 

 miscellaneous studies. 



Construction work has been limited chiefly to the erection of a '^dng 

 of the Pasadena laboratory to contain the 500-k. w. generator and its 

 switchboard; a large underground tank for the cooling system of the 

 new solenoid magnet; the completion of the observing platforms for 

 observers at the principal and Cassegrain foci of the Hooker telescope; 

 installation of the constant-temperature control system for the 100-inch 

 mirror; and the various additions to instrumental equipment aheady 

 mentioned. The Hooker telescope is now essentially complete, but 

 work on its accessory instruments will be continued as long as new and 

 promising devices are in view. 



